High-pressure sensors are used in numerous application areas in the automobile industry. For example, they are used in injection systems, anti-lock braking systems, driving dynamics control systems, and in other systems.
Conventional high-pressure sensors include a deformation member, also termed substrate, on which, using thin-layer technology, a layer system is deposited made up of an insulation layer, for example, composed of silicon oxide, a patterned functional layer, for example, made of poly-Si or NiCr, a contacting field and a passivation layer. The functional layer transforms a mechanical deformation of a diaphragm of the deformation member into an electrical signal.
The sensor, made up of the deformation member and the deposited thin layer, is subsequently assembled along with a pressure connection piece and an injection-molded housing, it is provided with contacts, and, if appropriate, it is furnished with evaluation electronics, thus constituting a complete high-pressure sensor.
One step in the manufacture of the high-pressure sensor is joining the pressure connection piece and the deformation member. This bond should be a tight seal, so that the pressure medium that is conveyed to the deformation member via the pressure connection piece does not escape through a leak in the welded seal. Furthermore, the bond should also be as free of stress as possible so that internal stresses of the deformation member caused by the welding process do not falsify its measuring signal and/or lead to an uncontrolled divergence with regard to sensitivity and response behavior in mass-produced pressure sensors.
At present, for welding pressure connection pieces and deformation members, the method of electron beam welding is generally used. This method is relatively expensive because it must be carried out in a vacuum, and it requires a great deal of processing time because, for each individual high-pressure sensor, the electron beam must be directed over its entire periphery in order to produce a closed welded seal. In this context, there also arises the problem that, as a consequence of the different thermal conditions at the beginning and at the end of the welding process, stresses are not distributed uniformly over the circumference of the welded seal. In addition, electron beam welding results in the production of burn-off, which must be removed in a time-consuming and costly manner.
A simple and cost-effective alternative to electron beam welding is resistance welding. However, this method has the disadvantage that to impress the welding current into the tool, a contact surface is required through which pressure is exerted on the object to be welded, and the surface cannot be too small in order to prevent an overheating of the contact area by the welding current that is supplied. To produce a contact area of this type in sufficient size, the deformation member is enlarged laterally, which results in a decrease in the yield of deformation members that could be manufactured from a wafer of a given size. In addition, it is believed that the number of deformation members that could be processed together in one coating procedure is smaller as a result, so that the cost reductions achieved during assembly by a simpler welding method are more than offset by the added expense in the manufacture of the deformation members.